Induction furnace



Aug. 17, 1937. w. ROHN 2,090,074

INDUCTION FURNACE Filed Jan. 12, 1935 I I I I p I I I l I E -IIIIII -II||I' 1mm Wilhelm Rofin Patented Aug. 17, 1937 UNITED STATES PATENT OFFICE INDUCTION FURNACE Wilhelm Rohn, Hanau-on-the-Main, Germany Application January 12, 1935, Serial No. 1,464 In Germany January 13, 1934 6 Claims.

ued in spirals beneath the bottom of a crucibleshaped lining in order to ensure as perfect an enclosure of the charge by the coil windings as possible. It has also been suggested to provide pancake coils above the lid and below the bottom as well as laterally at the mantle planes of rectangular linings. It is furthermore well-known in furnace practice to provide several layers of windings at the pancake as well as at the cylindrical coils. In every one of the above-named cases the various coils carry only one kind of current at a time.

Moreover it has already been proposed to provide a currentless cooling pipe system arranged in spirals between the bowl-shaped lining of induction furnaces operated by means of centered symmetrical pancake coils with medium or low frequency current and these pancake coils. The above-mentioned types of furnaces, if operated on a low frequency, are characterized by a very rapid movement of the bath which occasionally exceeds the permissible limits.

In order to avoid an excessive movement of the bath or to ensure better regulation of the eddies of the bath which, at least during certain operating stages, should be changed as far as possible into an absolutely uniform and more moderate eddy movement, it might be suggested to separate electrically the various parts of the coils from each other and to feed them with currents of different frequencies in order to be able to convey the main part of the energy with low frequency and only the smaller part with high frequency during operatingstages for which stirring is desired to be as strong as-p0ssible, and to proceed in the opposite way if but little movement of the bath is desired. The result of this modification would be that the windings, which are now independent of each other, influence each other inductively, thus causing trouble.

I have found it advisable to arrange the coils carrying currentsof different frequencies in such a manner that winding elements carrying different frequencies should as far as possible cross one another in angles of approximately 90". It proved to be expedient in this case to feed the inner coil with a higher frequency than the outer coil, and simultaneously to arrange the magnetic axes so that the high frequency axis stands vertically to the bath level and that the low frequency axis (or axes) has (or have) at least a considerable horizontal component.

Since the coil elements cross each other almost vertically on nearly the whole length of the winding, the troublesome mutual induction is eliminated in the case of these new coils. Care should, however, be taken that those parts of the outer coils which run horizontally andare thus in parallel with the windings of the inner coil, are kept as short aspossible. It will therefore be of advantage to wind the external coilsas seen in the elevationas narrow as possible with regard to their horizontal width; that is, the conductors of the external coils should lie, for the greater part of their length in a. vertical plane.

It has further proved to be advisable to subdivide the inner coil into a number of branches being electrically in parallel with each other and insulated from each other.

The advantages of the new arrangement are mainly metallurgical; the new coil combinations permit a hitherto impossible close regulation of the movement of the bath which is made more uniform over the whole charge surface if high frequency becomes predominant. It is also possible by this new measure to regulate in any way desired the depth of penetration of the effective magnetic field.

The drawing is an elevation of an advantageous embodiment of the invention in question, namely a coreless induction furnace. The hemi-spherical hearth l is surrounded by a bowl-shaped coil 2 with vertical axis 3. This coil 2 is operated on a 7 higher frequency and'is preferably designed as a hollow spiral suitable for the passage of a cooling medium. Outside this coil 2 there are arranged coils 4, possessing an axis at 5, that of the middle coil 4 standing almost vertically to the plane of the drawing. The coils 4, which in operation carry currents of low frequency, are distributed centrically and symmetrically around the circumference of the lining. The coils arranged to the right of the centre of the drawing have been omitted so that the bowl-shaped coil situated beneath the same becomes visible. The number .of coils with predominantly horizontal magnetic axis may, for instance, be three or a multiple of three, and these coils may then be connected to the three phases of a three-phase current.

This last-described arrangement assures simultaneously that the residual influence of the horizontally-arranged coil parts (which are in parallel with the winding plane of the other coil) is neutralized, as the total induction of the three phases on the horizontal convolutions of the other coil is zero.

for passing a current of lower frequency to said second induction coil. c

2. An induction furnace comprising a crucible for holding molten metal, an induction coil surl5 rounding said crucible, a second induction ,coil

superimposed upon said first induction coil, the .said second induction coil having its conductors for the most part crossing the conductors of said first coil at substantially right angles, there- 20 by substantially eliminating the mutual induction of said coils, means for passing alternating current of high frequency through one of said induction coils and means for passing alternating current of a lower frequency through the other 25 of said induction coils.

3. A coreless induction furnace having a ci' ucible for heating and stirring molten metal comprising at least two superimposed induction coils, the inner coil surrounding said crucible and havaoing a substantially vertical magnetic axis, the

outer coil being a pancake coil and having a magnetic axis having a large horizontal component, means for supplying high frequency alternating current to said inner coil, means for supplying 38 alternating current of lesser frequency to said outer coil and means for controlling the proportion of current supplied to said coils.

4. An induction furnace comprising a crucible for heating and stirring molten metal, an inducticn coil surrounding said crucible, a .plurality of polyphase induction coils arranged symmetrically around said crucible having windings which for the greater part cross the windings of said first induction coil at substantially right angles and means for supplying alternating currentof two different frequencies to said induction coils.

5. An induction furnace comprising a crucible for heating and stirring molten metal, an induction coil winding surrounding said crucible, a polyphase winding forming a plurality of induction coils, at least one coil for each phase, arranged symmetrically around said crucible, said polyphase induction coils having a short side parallel to the winding'of said first induction coil but having their windings for the most part crossing the winding of said first coil atsubstantial- 1y right angles, the mutual induction of said windings being substantially eliminated, and means for supplying alternating current of two different frequencies to said windings.

6. An induction furnace for heating and stir- =ring molten metal comprising a crucible free from induction elements passing therethrough,

an induction coil in inductive relationship with a bddy of molten metal within said crucible, a second induction coil also in inductive relationship with said body of molten metal, said coils having their magnetic axes substantially at right angles to each other and the conductors of one coil crossing the conductors of the other coil for the most part at substantially right angles,

W'ILHELM ROHN. 

